With the advent of general anesthesia, local anesthetic techniques in dogs and cats have all but disappeared from veterinary practice. Recently, local anesthetic techniques are again being popularized, this time in combination with general anesthesia. The reason for advocating both techniques is due to recent research concerning pain, analgesia and preemptive analgesia. This research has documented that long-term changes occur within the peripheral and central nervous systems following noxious input and that these contribute to the post-injury pain hypersensitivity state found postoperatively. These changes have been termed peripheral sensitization (a reduction in the threshold of nociceptor afferent peripheral terminals) and central sensitization (an activity-dependent increase in the excitability of spinal neurons). In patients, this manifests as an increase in the response to noxious stimuli and a decrease in the pain threshold, both at the site of injury and in the surrounding uninjured tissues. Recent evidence suggests that preoperative regional administration of local anesthetics can preempt postoperative pain by preventing the establishment of central sensitization.

LOCAL ANESTHETIC DRUGS

Local anesthetics are drugs that, when applied locally to nerve tissue (endings or fibers), cause reversible blockade of nerve impulse conduction. At effective concentrations, local anesthetics block transmission of autonomic, somatic sensory and somatic motor impulses. Thus, depending on the nerve and the area innervated, autonomic nervous system blockade, anesthesia, and/or skeletal muscle paralysis may result.

General properties

The typical local anesthetic molecule consists of an unsaturated aromatic group (imparting lipid solubility) linked by an intermediate chain to a tertiary amine end (imparting water solubility). The clinically important local anesthetics are divided into two distinct chemical groups based on their intermediate chain. The aminoesters contain an ester link, while the aminoamides contain an amide link, between the aromatic and amine ends. The agents most commonly administered in veterinary anesthesia are chloroprocaine, lidocaine, mepivacaine, and bupivacaine. All, except chloroprocaine, are aminoamides. Generally speaking, lidocaine and bupivacaine will suffice for most veterinary practice situations. The clinically important properties of the local anesthetics include potency, speed of onset, duration of anesthetic action, and differential sensory/motor blockade. The clinically observed rates of onset and recovery from blockade are governed by the relatively slow diffusion of local anesthetic molecules into and out of the whole nerve. While, in vitro, these depend on the physicochemical properties of the different agents, in vivo, dose and concentration also affect onset of action. For example, 0.25% bupivacaine has a slow onset of action, but increasing the concentration to 0.75% results in a significant acceleration of anesthetic effect. Generally, lidocaine is considered to have a rapid onset of action (10–15 minutes), while bupivacaine has an intermediate onset of action (20–30 minutes). Lidocaine has a moderate duration of action (60–120 minutes) while bupivacaine has the longest duration (three to eight hours). Lidocaine is one of the most widely used local anesthetics and is available in concentrations from 0.5 to 5.0% with or without epinephrine, as well as in gel and ointment preparations. It can be used for all forms of regional anesthesia. Bupivacaine is available as 0.25, 0.5 and 0.75% solutions and is used most commonly for all forms of regional anesthesia except topical. Bupivacaine demonstrates significant separation of sensory and motor blockade, meaning that it is able to produce adequate antinociception without profound inhibition of motor activity, particularly when dilute solutions are employed. For this reason, dilute bupivacaine is the choice in people for obstetrical anesthesia.

Mechanism of action

Local anesthetics inhibit the generation and propagation of nerve impulses by blockade of voltage-gated sodium channels in the nerve membrane. Local anesthetics are capable of blocking all nerves, thus their action is not limited to the usually more desirable sensory block but also motor loss. However, nerve fibers differ substantially in their susceptibility to local anesthetic blockade due to size and, presence and absence of myelin.

Factors influencing anesthetic activity

As the dosage of local anesthetic is increased, the probability and duration of satisfactory anesthesia increase, and the time to onset is shortened. The dosage of local anesthetic can be increased by administering either a larger volume or a more concentrated solution.

Vasoconstrictors, usually epinephrine (5 µg/mL or 1:200,000), are frequently included in local anesthetic solutions to decrease the rate of vascular absorption, thereby allowing more anesthetic molecules to reach the nerve membrane and so improve the depth and duration of anesthesia. The extent of this effect depends on the specific local anesthetic and the site of injection. Generally, epinephrine significantly extends the duration of both infiltration anesthesia and peripheral nerve blocks with many agents, such as lidocaine.

Site of injection also influences local anesthetic activity, with the most rapid onset but the shortest duration occurring following spinal or subcutaneous administration of local anesthetics. The longest latencies and durations are observed following brachial plexus block.

LOCAL ANESTHETIC TECHNIQUES

Local anesthetics are most often used to produce regional anesthesia (implying that a region rather than the entire body is affected). Regional anesthesia is usually divided into topical anesthesia, local infiltration, peripheral nerve block, intra-articular administration, intravenous block, epidural block and spinal (subarachnoid) block. Since epidural and spinal blocks form the basis of another lecture, this talk will exclude these two categories of regional anesthesia. All local anesthetic agents can induce systemic toxicity manifested as convulsions followed by cardiac arrest. To prevent this, total doses administered by the various techniques need to be calculated and maximal safe doses known. As a general rule, the total infiltrative dose of lidocaine should be < 8 mg/kg in dogs and < 4 mg/kg in cats, while the dose of bupivacaine should be < 2 mg/kg in both dogs and cats.

Topical anesthesia

Lidocaine is effective when placed topically on mucous membranes and maybe used in the mouth, tracheobronchial tree, esophagus and genitourinary tract. Compared to infiltration, the time for onset of action is generally longer and the analgesia less; for example, topical application on mucous membranes penetrates to a depth of 2 mm and anesthesia lasts 15–20 minutes. The cream most commonly used on skin contains a 5% eutectic mixture of lidocaine and prilocaine (EMLA), which penetrates the cornified skin barrier within 1 hour of topical application. In people, topical application of lidocaine to wound edges was effective in reducing postoperative pain and the effect outlasted the expected duration of action of the drugs.(1,2) In dogs, bupivacaine solution left in contact with the surgical site for 20 minutes (splash block) was not found to be beneficial in dogs undergoing ear ablation.(3) In cats, wound irrigation with bupivacaine prior to wound closure following onychectomy did not reduce postoperative pain compared with saline treated controls.(4) Interpleural administration of local anesthetic is advocated as a technique for providing postoperative analgesia following thoracotomy. Its popularity in people has waxed and waned and its efficacy is variable probably because of technical problems related to blood in the pleural space, chest tube drainage and pleural disease. Evaluation of this technique in dogs after intercostal thoracotomy demonstrated better pulmonary function and a more comfortable and quieter recovery compared to dogs treated with intramuscular morphine.(5)

Local infiltration

Local infiltration requires extravascular placement by direct injection. The solution is placed by intradermal or subcutaneous injection using a 22 to 25-gauge needle and is slowly injected advancing the needle along the line of the proposed incision. The amount of anesthetic required depends on the size of the area to be anesthetized, the drug and the size of the patient. Generally, the lowest possible dose that will produce the desired effect should be administered. If a large area needs to be anesthetized, the local anesthetic may need to be diluted to prevent toxicity. Local anesthetics containing epinephrine should not be injected into tissues supplied by end arteries, such as phalanges, ears and tail. In conscious patients, subcutaneous administration is often painful, due in part to the acidic nature of the solution. Addition of sodium bicarbonate will decrease pain and hasten onset.

Peripheral nerve blocks

Injection of a local anesthetic solution into the connective tissue surrounding a particular nerve produces loss of sensation (sensory block) and/or paralysis (motor block) in the region supplied by the nerve. Smaller volumes of drugs are needed to produce the block, reducing the danger of systemic toxicity.

Head

Useful nerve blocks for dental and other procedures involving the maxilla and mandible include infiltration of the infraorbital, mandibular and mental nerves. The infraorbital nerve is blocked at its point of emergence from the infraorbital canal. The needle is inserted either intraorally or extraorally 1 cm cranial to the bony lip of the infraorbital foramen. The infraorbital foramen can be palpated between the dorsal border of the zygomatic process and the gingival margin of the second/third premolar teeth. The needle is advanced into the infraorbital foramen. The area innervated by this nerve includes the upper lip and nose, roof of the nasal cavity, and the upper incisors, canine, premolars and first molar teeth.(6) The mandibular nerve is blocked either intraorally or extraorally at the point of entry of the nerve into the mandibular foramen. If an intraoral approach is used the mandibular nerve can be palpated as it runs ventrally along the vertical ramus of the mandible. The nerve is palpated with the index finger of one hand and the syringe with needle attached is introduced with the other hand. If an extraoral approach is used, the nerve is again palpated with the index finger while the needle is inserted at the lower angle of the mandible rostral to the angular process. The needle is advanced against the medial surface of the ramus until it can be felt under the index finger. The area innervated by this nerve includes the incisors, canine, premolars, molars, and skin and mucosa of the chin and lower lip. The middle mental nerve can be blocked as it exits the middle mental foramen level with the second lower premolar. If the needle is advanced into the mental foramen, the area blocked includes the lower lip and the incisors, canine, premolars and first molar.(6)

Forelimb

The brachial plexus block is suitable for surgery on the front limb within or distal to the elbow. A spinal needle 22 to 20-gauge, 2.5–3.5 inches is suitable. The needle is inserted at the point of the shoulder, medial to the shoulder joint and directed towards the costochondral junction making sure to stay external to the thoracic cavity. The needle should be inserted until the tip is just caudal to the first rib and a line block is performed as the needle is withdrawn. Local anesthetic is deposited in close proximity to the radial, median, ulnar, musculocutaneous, and axillary nerves.

Alternatively, for surgery distal to the elbow, the individual nerves (median, ulnar, musculocutaneous and radial) may be infiltrated for analgesia below the elbow. The first three nerves can all be blocked together by approaching the medial side of the foreleg at the level of distal and middle thirds of the humerus between the biceps and the medial head of the triceps. The superficial branches of the radial nerve can be blocked as they pass over the dorsal surface of the elbow, where they lie next to the cephalic vein.

For onychectomy, nerve blocks of the superficial radial, the dorsal cutaneous and palmar branches of the ulnar nerve and the median nerve are a very useful procedure that not only facilitates surgery, but also results in good postoperative analgesia.

Thorax

Intercostal nerve blocks are administered to reduce the amount of other analgesics needed to treat postoperative pain and to improve postoperative pulmonary function following lateral thoracotomy. Intercostal nerve blocks can be performed prior to surgery or intraoperatively, either during or after thoracotomy. A minimum of two adjacent intercostal spaces both cranial and caudal to the incision must be blocked because of the overlap of nerve supply. The site for needle placement is the caudal border of the rib near the intervertebral foramen.

Hindlimb

Anesthesia to the hindlimb distal to the hip can be achieved by selective blockade of the saphenous, common peroneal and tibial nerves. The superficial and deep fibular nerves and the tibial nerve can be blocked for procedures involving the foot.

Intra-articular administration

Most studies indicate that intra-articular bupivacaine is an effective method for providing postoperative analgesia. When compared with controls these patients require less postoperative pain medication (at least over the first 24 hours) and are able to get up and mobilize the joint sooner. The effectiveness of intra-articular bupivacaine has been confirmed in dogs.(7)

Intravenous block

Intravenous regional anesthesia (Bier block) produces anesthesia in the distal portion of the limb. An intravenous catheter is placed in either the cephalic or saphenous vein depending on the limb to be blocked. The limb is then exsanguinated by wrapping it with an Esmarch bandage and a rubber tourniquet is placed proximal to the bandage and the bandage unwrapped. Lidocaine 2.5–5.0 mg/kg is injected intravenously under light pressure. Maximal anesthesia is achieved in 5-10 minutes.

References

1.  Casey WF, Rice LJ, Hannalah RF, et al. A comparison between bupivacaine instillation versus ilioinguinal/iliohypogastric nerve block for postoperative analgesia following inguinal herniorrhaphy in children. Anesthesiology 1990;72:637-639.

2.  Sinclair R, Cassuto J, Hogstrom S, et al. Topical anesthesia with lidocaine aerosol in the control of postoperative pain. Anesthesiology 1988;68:895-901.

3.  Buback JL, Boothe HW, Carroll GL, et al. Comparison of three methods for relief of pain after ear canal ablation in dogs. Vet Surg 1996;25:380-385.

4.  Winkler KP, Greenfield, CL, Benson, GJ. The effect of wound irrigation with bupivacaine on postoperative analgesia of the feline onychectomy patient. JAAHA 1997;33:346-352.

5.  Stobie D, Caywood DD, Rozanski EA, et al. Evaluation of pulmonary function and analgesia in dogs after intercostal thoracotomy and use of morphine administered intramuscularly or intrapleurally and bupivacaine administered intrapleurally. Am J Vet Res 1995;56:1098-1109.

6.  Gross ME, Pope ER, O’Brien D, et al. Regional anesthesia of the infraorbital and inferior alveolar nerves during noninvasive tooth pulp stimulation in halothane-anesthetized dogs. J Amer Vet Med Assoc 1997;11:1403-1405.

7.  Sammarco JL, Conzemius MG, Perkowski SZ, et al. Postoperative analgesia for stifle surgery: A comparison of intra-articular bupivacaine, morphine, or saline. Vet Surg 1996;25:9-69.